The process of signal transduction often involves proteins that have extracellular domains, transmembrane domains, and intracellular domains. During ligand binding there is often oligomerization of receptor molecules in order to transmit effectively the signal to the intracellular component of the cell. The immune system is an excellent example of a signal transduction pathway that works by these methods (Rosen et al. J. Med. Chem. 38: 48-55).
The immune system is a defense system found in most advanced forms of higher vertebrates. A properly functioning lymphatic and immune system distinguishes between self and nonself. A healthy body protects against foreign invaders, such as bacteria, viruses, fungi, and parasites. As the body encounters foreign material (nonself), also known as an antigens, the immune system becomes activated. An antigen is recognized by characteristic shapes or epitopes on its surface. This defense mechanism provides a means of rapid and highly specific responses that are used to protect an organism against invasion by pathogenic microorganisms. It is the myriad of pathogenic microorganisms that have principally caused the evolution of the immune system to its current form. In addition to protection against infectious agents, specific immune responses are thought to be involved in surveillance against alterations in self antigens as seen in tumor development. Immune responses are also involved in the development of autoimmune disease, AIDS, as well as rejection of transplanted tissues.